The subfloor pictured was laid atop existing 3/4-inch plywood to ensure a smooth substrate surface for a hardwood installation.

The success and longevity of any floor covering installation largely rests on an adequate subfloor and the installer's responsibility for qualifying and preparation. Any hard-surface installation depends upon the support of an approved subfloor. Carpeting installed over cushion usually can grant a "pardon" to just about any subfloor. On the other hand, carpeting can also hide a multitude of sins and may be considered the flooring industry's greatest cover-up story.

With the exception of concrete, both resilient and hard-tile products will require an additional underlayment specified by the manufacturers. With regard to the hardwood flooring industry, engineered products that are installed over a wood-type subfloor with an adhesive normally require an additional underlayment to establish a smooth and flat surface.

In today's ever-changing construction industry, the importance of a qualified underlayment and/or subfloor remains paramount in the eyes of the hardwood flooring industry. New construction has always been considered cut and dried in determining the suitability of the subfloor. Yes, there still may be challenges in floor preparation, but at least they are exposed upon your arrival at the job site. On the other side, the remodel/replacement market may reveal your very own fear factor once hidden subfloor inadequacies are exposed.

The replacement market continues to be characterized by homeowners' strong desire to replace existing floor coverings in just about any area of the home with tried-and-true hardwood flooring. For instance, most kitchens over a suspended subfloor typically featured resilient floor installations over an additional recommended underlayment. These are considered an excellent base for almost any type of hardwood flooring installation. Resilient flooring serves as an excellent moisture barrier as well. In the case of engineered flooring over a concrete slab with existing resilient, check with the adhesive manufacturer for adhesion compatibility.

The replacement market for hardwood flooring installations is capable of throwing more curves at the contractor than a major league pitcher. In addition to on-grade and suspended-floor installations, homes constructed with crawlspaces have their very own distinct oddities.

In these installations, a black polyethylene sheet between 6 to 8 mils in thickness must be intact over the ground below with a minimum 24 inches of clearance between the barrier and the subfloor. As we all know, excessive moisture will cause movement and potential swelling in the subfloor. In addition, adequate cross ventilation is a must and, in some cases, a fan for air circulation should be in place and operating. A crawlspace should have 1.5 percent of open venting per 1,000 square feet of area.

Protecting the subfloor from below is just as critical as the environment above is to the hardwood flooring installation itself. Damp or leaky basements create excessive humidity. The resulting moisture first penetrates the subfloor and eventually migrates into the hardwood flooring to cause swelling or other problems.

Pictured is the transition of subfloor to existing terazzo. This requires special consideration on the installer's part to ensure the subfloor fits flush with the terazzo.
Another issue of concern relates to the term "level" vs. the word "flat." Hardwood flooring contractors who refer to "level" in their installation contracts may want to give some thought to becoming an excavating contractor. You may never become one physically, but mentally, in the eyes of the homeowner, your binding contract makes you one.

The fastest way I know to "dig holes" from which you can't escape is to use the distinct description of "leveling" the subfloor and/or hardwood installation. The industry guidelines refer to the mandate that the subfloor must be "flat, clean and dry." I would strongly recommend that you omit the term "level" from your contracts and any verbal communication with the consumer/homeowner.

Let's explore some up the potential pitfalls that you may encounter with an existing subfloor and consider some suggested remedies.

Subfloor appears to have been exposed to water. Examine the floor for softness or delamination. Also check the moisture content of the subfloor. Acceptable tolerance should be no greater than 4 percent between the subfloor and the hardwood flooring. If the percentage exceeds that tolerance, chances are that a water leak may still exist or additional time for drying out may be required.

Drive a no. 8 common nail into the area and then try and remove it. If the nail exhibits little resistance in driving or pulling, replace the area. Avoid patching and be sure that the all edges of the plywood rest on a floor joist. Use CDX exterior-grade plywood only and make certain that the thickness is the same as the adjacent plywood. For any hardwood flooring installation, a minimum thickness of 5/8 inch is required but 3/4 inch is preferred. Please note that several OSB subfloors are now APA approved as well.

Subfloor has squeaks and/or high spots at the joists. Walk the entire subfloor thoroughly and mark areas in need of additional fasteners. Locate the squeaks and secure the subfloor to the floor joist with 2-inch decking screws. Recheck for squeaks.

If you identify a high spot over a joist after checking for flatness with a chalk line, the floor may have what is called a "crowned" joist. Crowned joists can be hard to correct. Some high joists can be corrected by rough sanding the subfloor directly over the high area. If the crown is minimal, utilize roofing shingles to taper off the crown to the next joist.

Also if the floor appears to be sagging, inspect from the below and drive wood wedges between the joist and the subfloor to elevate the sagging area. If the subfloor appears to have a downward roll toward the wall line, utilize 15 lb. roofing paper in tapered layers, or shingles again, depending upon the drop in the subfloor.

Concrete subfloor. Check for holes, cracks and dips in the concrete. First, always conduct a moisture test at several locations. If moisture content is acceptable, proceed with floor preparation.

Fill any holes, cracks and or dips with a leveling compound. In the event there are high spots, grind them down with 20-grit sandpaper. Remove any foreign substances as well. Avoid using solvents to clean the concrete to eliminate the possibility of flash back and/or combustion. A properly prepared concrete subfloor should be flat, sound and clean before the installation may proceed.

As you can "see," if you really care to take a "look," subfloors are a key part of your installation. Wood or Wood Knot only asks that you just keep your eyes "peeled" and your mind "opened."